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OIT and Indirect Illumination using DX11 Linked Lists

OIT and Indirect Illumination using DX11 Linked Lists. Holger Gruen AMD ISV Relations Nicolas Thibieroz AMD ISV Relations. Agenda. Introduction Linked List Rendering Order Independent Transparency Indirect Illumination Q&A. Introduction.

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OIT and Indirect Illumination using DX11 Linked Lists

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  1. OIT and Indirect Illumination using DX11 Linked Lists Holger Gruen AMD ISV Relations Nicolas Thibieroz AMD ISV Relations

  2. Agenda • Introduction • Linked List Rendering • Order Independent Transparency • Indirect Illumination • Q&A

  3. Introduction • Direct3D 11 HW opens the door to many new rendering algorithms • In particular per pixel linked lists allow for a number of new techniquesOIT, Indirect Shadows, Ray Tracing of dynamic scenes, REYES surface dicing, custom AA, Irregular Z-buffering, custom blending, Advanced Depth of Field, etc. • This talk will walk you through:A DX11 implementation of per-pixel linked list and two effects that utilize this techique • OIT • Indirect Illumination

  4. Per-Pixel Linked Lists with Direct3D 11 Nicolas Thibieroz European ISV Relations AMD Element Element Element Element Link Link Link Link

  5. Why Linked Lists? • Data structure useful for programming • Very hard to implement efficiently with previous real-time graphics APIs • DX11 allows efficient creation and parsing of linked lists • Per-pixel linked lists • A collection of linked lists enumerating all pixels belonging to the same screen position Element Element Element Element Link Link Link Link

  6. Two-step process • 1) Linked List Creation • Store incoming fragments into linked lists • 2) Rendering from Linked List • Linked List traversal and processing of stored fragments

  7. Creating Per-Pixel Linked Lists

  8. PS5.0 and UAVs • Uses a Pixel Shader 5.0 to store fragments into linked lists • Not a Compute Shader 5.0! • Uses atomic operations • Two UAV buffers required • - “Fragment & Link” buffer • - “Start Offset” buffer UAV = Unordered Access View

  9. Fragment & Link Buffer • The “Fragment & Link” buffer contains data and link for all fragments to store • Must be large enough to store all fragments • Created with Counter support • D3D11_BUFFER_UAV_FLAG_COUNTER flag in UAV view • Declaration: structFragmentAndLinkBuffer_STRUCT { FragmentData_STRUCTFragmentData; // Fragment data uintuNext; // Link to next fragment }; RWStructuredBuffer <FragmentAndLinkBuffer_STRUCT> FLBuffer;

  10. Start Offset Buffer • The “Start Offset” buffer contains the offset of the last fragment written at every pixel location • Screen-sized:(width * height * sizeof(UINT32) ) • Initialized to magic value (e.g. -1) • Magic value indicates no more fragments are stored (i.e. end of the list) • Declaration: RWByteAddressBufferStartOffsetBuffer;

  11. Linked List Creation (1) • No color Render Target bound! • No rendering yet, just storing in L.L. • Depth buffer bound if needed • OIT will need it in a few slides • UAVs bounds as input/output: • StartOffsetBuffer (R/W) • FragmentAndLinkBuffer (W)

  12. Linked List Creation (2a) Start Offset Buffer -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 Viewport -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 0 Fragment and Link Buffer Counter = Fragment and Link Buffer Fragment and Link Buffer Fragment and Link Buffer

  13. Linked List Creation (2b) Start Offset Buffer -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 Viewport -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 1 0 Fragment and Link Buffer Counter = Fragment and Link Buffer -1 Fragment and Link Buffer -1 Fragment and Link Buffer

  14. Linked List Creation (2c) Start Offset Buffer -1 -1 -1 -1 -1 -1 -1 0 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 Viewport -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 1 3 2 Fragment and Link Buffer Counter = Fragment and Link Buffer -1 Fragment and Link Buffer -1 -1 Fragment and Link Buffer

  15. Linked List Creation (2d) Start Offset Buffer -1 -1 -1 -1 -1 -1 -1 0 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 Viewport -1 -1 -1 -1 -1 -1 -1 -1 -1 1 2 -1 -1 -1 -1 -1 -1 -1 4 3 5 Fragment and Link Buffer Counter = -1 -1 -1 0 Fragment and Link Buffer -1 Fragment and Link Buffer

  16. Linked List Creation - Code float PS_StoreFragments(PS_INPUT input) : SV_Target { // Calculate fragment data (color, depth, etc.) FragmentData_STRUCTFragmentData = ComputeFragment(); // Retrieve current pixel count and increase counter uintuPixelCount = FLBuffer.IncrementCounter(); // Exchange offsets in StartOffsetBuffer uintvPos = uint(input.vPos); uintuStartOffsetAddress= 4 * ( (SCREEN_WIDTH*vPos.y) + vPos.x ); uintuOldStartOffset; StartOffsetBuffer.InterlockedExchange(uStartOffsetAddress, uPixelCount, uOldStartOffset); // Add new fragment entry in Fragment & Link Buffer FragmentAndLinkBuffer_STRUCT Element; Element.FragmentData = FragmentData; Element.uNext = uOldStartOffset; FLBuffer[uPixelCount] = Element; }

  17. Traversing Per-Pixel Linked Lists

  18. Rendering Pixels (1) • “Start Offset” Buffer and “Fragment & Link” Buffer now bound as SRV Buffer<uint> StartOffsetBufferSRV; StructuredBuffer<FragmentAndLinkBuffer_STRUCT> FLBufferSRV; • Render a fullscreen quad • For each pixel, parse the linked list and retrieve fragments for this screen position • Process list of fragments as required • Depends on algorithm • e.g. sorting, finding maximum, etc. SRV = Shader Resource View

  19. Rendering from Linked List Start Offset Buffer -1 -1 -1 -1 -1 -1 -1 3 3 4 -1 -1 -1 -1 -1 -1 -1 -1 -1 Render Target -1 -1 -1 -1 -1 -1 -1 -1 -1 1 2 -1 -1 -1 -1 -1 -1 -1 Fragment and Link Buffer -1 -1 -1 -1 0 0 Fragment and Link Buffer -1 Fragment and Link Buffer

  20. Rendering Pixels (2) float4 PS_RenderFragments(PS_INPUT input) : SV_Target { // Calculate UINT-aligned start offset buffer address uintvPos = uint(input.vPos); uintuStartOffsetAddress = SCREEN_WIDTH*vPos.y + vPos.x; // Fetch offset of first fragment for current pixel uintuOffset = StartOffsetBufferSRV.Load(uStartOffsetAddress); // Parse linked list for all fragments at this position float4 FinalColor=float4(0,0,0,0); while (uOffset!=0xFFFFFFFF) // 0xFFFFFFFF is magic value { // Retrieve pixel at current offset Element=FLBufferSRV[uOffset]; // Process pixel as required ProcessPixel(Element, FinalColor); // Retrieve next offset uOffset = Element.uNext; } return (FinalColor); }

  21. Order-Independent Transparency via Per-Pixel Linked Lists Nicolas Thibieroz European ISV Relations AMD

  22. Description • Straight application of the linked list algorithm • Stores transparent fragments into PPLL • Rendering phase sorts pixels in a back-to-front order and blends them manually in a pixel shader • Blend mode can be unique per-pixel! • Special case for MSAA support

  23. Linked List Structure • Optimize performance by reducing amount of data to write to/read from UAV • E.g. uint instead of float4 for color • Example data structure for OIT: structFragmentAndLinkBuffer_STRUCT { uintuPixelColor; // Packed pixel color uintuDepth; // Pixel depth uintuNext; // Address of next link }; • May also get away with packed color and depth into the same uint! (if same alpha) • 16 bits color (565) + 16 bits depth • Performance/memory/quality trade-off

  24. Visible Fragments Only! • Use [earlydepthstencil] in front of Linked List creation pixel shader • This ensures only transparent fragments that pass the depth test are stored • i.e. Visible fragments! • Allows performance savings and rendering correctness! [earlydepthstencil] float PS_StoreFragments(PS_INPUT input) : SV_Target { ... }

  25. Sorting Pixels • Sorting in place requires R/W access to Linked List • Sparse memory accesses = slow! • Better way is to copy all pixels into array of temp registers • Then do the sorting • Temp array declaration means a hard limit on number of pixel per screen coordinates • Required trade-off for performance

  26. Sorting and Blending 0.95 0.93 0.87 0.98 • Blend fragments back to front in PS • Blending algorithm up to app • Example: SRCALPHA-INVSRCALPHA • Or unique per pixel! (stored in fragment data) • Background passed as input texture • Actual HW blending mode disabled Background color Temp Array Render Target PS color 0.98 0.95 0.87 0.93 -1 34 12 0

  27. Storing Pixels for Sorting (...) static uint2 SortedPixels[MAX_SORTED_PIXELS]; // Parse linked list for all pixels at this position // and store them into temp array for later sorting intnNumPixels=0; while (uOffset!=0xFFFFFFFF) { // Retrieve pixel at current offset Element=FLBufferSRV[uOffset]; // Copy pixel data into temp array SortedPixels[nNumPixels++]= uint2(Element.uPixelColor, Element.uDepth); // Retrieve next offset [flatten]uOffset = (nNumPixels>=MAX_SORTED_PIXELS) ? 0xFFFFFFFF : Element.uNext; } // Sort pixels in-place SortPixelsInPlace(SortedPixels, nNumPixels); (...)

  28. Pixel Blending in PS (...) // Retrieve current color from background texture float4 vCurrentColor=BackgroundTexture.Load(int3(vPos.xy, 0)); // Rendering pixels using SRCALPHA-INVSRCALPHA blending for (int k=0; k<nNumPixels; k++) { // Retrieve next unblended furthermost pixel float4 vPixColor= UnpackFromUint(SortedPixels[k].x); // Manual blending between current fragment and previous one vCurrentColor.xyz= lerp(vCurrentColor.xyz, vPixColor.xyz, vPixColor.w); } // Return manually-blended color return vCurrentColor; }

  29. OIT via Per-Pixel Linked Lists with MSAA Support

  30. Sample Coverage • Storing individual samples into Linked Lists requires a huge amount of memory • ... and performance will suffer! • Solution is to store transparent pixels into PPLL as before • But including sample coverage too! • Requires as many bits as MSAA mode • Declare SV_COVERAGE in PS structure struct PS_INPUT { float3 vNormal : NORMAL; float2 vTex : TEXCOORD; float4 vPos : SV_POSITION; uintuCoverage : SV_COVERAGE; }

  31. Linked List Structure • Almost unchanged from previously • Depth is now packed into 24 bits • 8 Bits are used to store coverage structFragmentAndLinkBuffer_STRUCT { uintuPixelColor; // Packed pixel color uintuDepthAndCoverage; // Depth + coverage uintuNext; // Address of next link };

  32. Sample Coverage Example • Third sample is covered • uCoverage = 0x04 (0100 in binary) Element.uDepthAndCoverage = ( In.vPos.z*(2^24-1) << 8 ) | In.uCoverage; Pixel Center Sample

  33. Rendering Samples (1) • Rendering phase needs to be able to write individual samples • Thus PS is run at sample frequency • Can be done by declaring SV_SAMPLEINDEX in input structure • Parse linked list and store pixels into temp array for later sorting • Similar to non-MSAA case • Difference is to only store sample if coverage matches sample index being rasterized

  34. Rendering Samples (2) static uint2 SortedPixels[MAX_SORTED_PIXELS]; // Parse linked list for all pixels at this position // and store them into temp array for later sorting intnNumPixels=0; while (uOffset!=0xFFFFFFFF) { // Retrieve pixel at current offset Element=FLBufferSRV[uOffset]; // Retrieve pixel coverage from linked list element uintuCoverage=UnpackCoverage(Element.uDepthAndCoverage); if ( uCoverage & (1<<In.uSampleIndex) ) { // Coverage matches current sample so copy pixel SortedPixels[nNumPixels++]=Element; } // Retrieve next offset [flatten]uOffset = (nNumPixels>=MAX_SORTED_PIXELS) ? 0xFFFFFFFF : Element.uNext; }

  35. DEMO OIT Linked List Demo

  36. Holger GruenEuropean ISV Relations AMD Direct3D 11 Indirect Illumination

  37. Indirect Illumination Introduction 1 • Real-time Indirect illumination is an active research topic • Numerous approaches existReflective Shadow Maps (RSM) [Dachsbacher/Stammiger05]Splatting Indirect Illumination [Dachsbacher/Stammiger2006]Multi-Res Splatting of Illumination [Wyman2009]Light propagation volumes [Kapalanyan2009]Approximating Dynamic Global Illumination in Image Space [Ritschel2009] Only a few support indirect shadowsImperfect Shadow Maps [Ritschel/Grosch2008]Micro-Rendering for Scalable, Parallel Final Gathering(SSDO) [Ritschel2010] Cascaded light propagation volumes for real-time indirect illumination [Kapalanyan/Dachsbacher2010] • Most approaches somehow extend to multi-bounce lighting

  38. Indirect Illumination Introduction 2 • This section will coverAn efficient and simple DX9-compliant RSM based implementation for smooth one bounce indirect illumination • Indirect shadows are ignored here • A Direct3D 11 technique that traces rays to compute indirect shadows • Part of this technique could generally be used for ray-tracing dynamic scenes

  39. Indirect Illumination w/o Indirect Shadows • Draw scene g-buffer • Draw Reflective Shadowmap (RSM) • RSM shows the part of the scene that recieves direct light from the light source • Draw Indirect Light buffer at ¼ res • RSM texels are used as light sources on g-buffer pixels for indirect lighting • Upsample Indirect Light (IL) • Draw final image adding IL

  40. Step 1 • G-Buffer needs to allow reconstruction of • World/Camera space position • World/Camera space normal • Color/ Albedo • DXGI_FORMAT_R32G32B32A32_FLOAT positions may be required for precise ray queries for indirect shadows

  41. Step 2 • RSM needs to allow reconstruction of • World space position • World space normal • Color/ Albedo • Only draw emitters of indirect light • DXGI_FORMAT_R32G32B32A32_FLOAT position may be required for ray precise queries for indirect shadows

  42. Step 3 • Render a ¼ res IL as a deferred op • Transform g-buffer pix to RSM space • ->Light Space->project to RSM texel space • Use a kernel of RSM texels as light sources • RSM texels also called Virtual Point Light(VPL) • Kernel size depends on • Desired speed • Desired look of the effect • RSM resolution

  43. Computing IL at a G-buf Pixel 1 Sum up contribution of all VPLs in the kernel

  44. Computing IL at a G-buf Pixel 2 RSM texel/VPL g-buffer pixel This term is very similar to terms used in radiosity form factor computations

  45. Computing IL at a G-buf Pixel 3 A naive solution for smooth IL needs to consider four VPL kernels with centers at t0, t1, t2 and t3. stx : sub RSM texel x position [0.0, 1.0[ sty : sub RSM texel y position [0.0, 1.0[

  46. Computing IL at a g-buf pixel 4 IndirectLight = (1.0f-sty) * ((1.0f-stx) * + stx * ) + (0.0f+sty) * ((1.0f-stx) * + stx * ) Evaluation of 4 big VPL kernels is slow  VPL kernel at t0 stx : sub texel x position [0.0, 1.0[ VPL kernel at t2 sty : sub texel y position [0.0, 1.0[ VPL kernel at t1 VPL kernel at t3

  47. Computing IL at a g-buf pixel 5 SmoothIndirectLight = (1.0f-sty)*(((1.0f-stx)*(B0+B3)+stx*(B2+B5))+B1)+ (0.0f+sty)*(((1.0f-stx)*(B6+B3)+stx*(B8+B5))+B7)+B4 stx : sub RSM texel x position of g-buf pix [0.0, 1.0[ sty : sub RSM texel y position of g-buf pix [0.0, 1.0[ This trick is probably known to some of you already. See backup for a detailed explanation !

  48. Indirect Light Buffer

  49. Step 4 • Indirect Light buffer is ¼ res • Perform a bilateral upsampling step • SeePeter-Pike Sloan, Naga K. Govindaraju, Derek Nowrouzezahrai, John Snyder. "Image-Based Proxy Accumulation for Real-Time Soft Global Illumination". Pacific Graphics 2007 • Result is a full resolution IL

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